1. Field of the Invention
The present invention relates to a concentration measuring device.
2. Description of the Related Art
There is known a concentration measuring device that measures a concentration of a test substance in a gas. For example, a device disclosed in Patent Literature 1 measures an amount of a petroleum-based hydrocarbon contained in soil by detecting the concentration of carbon dioxide generated by the combustion of the hydrocarbon. The concentration measuring device includes: a heater for heating a mixed gas containing oxygen and nitrogen; a soil case for holding soil; a catalyst case for holding a catalyst; and a concentration sensor for measuring a concentration of carbon dioxide.
When the soil is exposed to the heated mixed gas in the device disclosed in Patent Literature 1, the hydrocarbon component contained in the soil is vaporized. Furthermore, when the vaporized hydrocarbon component comes into contact with the catalyst, predetermined amounts of water and carbon dioxide are generated due to a reaction of the hydrocarbon component with the oxygen in the mixed gas. Since the reaction using the catalyst is known, the concentration of the hydrocarbon component contained in the initial soil can be calculated from the concentration of carbon dioxide detected by the concentration sensor. Finally, the gas having passed through a concentration detection chamber housing the concentration sensor is discharged directly into outside air.
Patent Literature 1: Japanese Patent Application Laid-open No 2011-232333 is introduced as the Prior Art Document.
In a case where the device described in Patent Literature 1 is used for a gas having a test substance in a low concentration, however, the concentration of carbon dioxide generated by a reaction with a catalyst may fall below a concentration range detectable by the concentration sensor. If a volume flow of the gas is increased in the vicinity of the concentration sensor, for example, the concentration detected by the concentration sensor is decreased. Thus, it is impossible to measure the concentration of carbon dioxide if it falls below the concentration range measurable by the concentration sensor. Therefore, in the device of Patent Literature 1, a level of accuracy required in gas flow control becomes high. Such a problem can be solved tentatively by employing a highly-sensitive concentration sensor. However, such a highly-sensitive concentration sensor has disadvantages such that it is expensive and handling thereof such as calibration is not easy.